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Discussion Starter #1
Ok, so i've banged out many clutchless upshifts/downshifts...mucho fun.
I'm part gear head though. What exactly is the reason why you can do this on a sequential trans but not the 5-spd in my car?
 

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ive done it with my subaru WRX many times.. The only problem i had was its not as responsive as chopping the throttle on a bike so it takes practice to rev match and and get it in smoothly..
 

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The site that gpz1100 referred to explains the baulk-ring synchromesh that most car gearboxes use. It makes normal clutched shifting easier, but at least in the case of VW manual gearboxes, it makes clutchless shifts extremely difficult because unless the RPM is *exactly matched* it won't let the next gear go in. I had a clutch linkage let go in my old car, and I had to drive it to a friend's shop without being able to disengage the clutch ... I did it by cranking and starting the engine in 1st gear and then rev-matching to get it into 3rd, or 2nd, or any higher gear that I could coax it into. I stopped by shifting to neutral and stopping the engine to be ready for the next crank-and-start in first.

Bike gearboxes are dog-engagement. Each gear has 3 pins on it that, when engaged, drive the adjacent gear via 3 slots in the adjacent gear. There is deliberately a lot of backlash (rotational clearance) between the dogs and the slots (something not found in synchromesh gearboxes). The purpose is so that during a higher-rpm shift, eventually the pin will go into the slot far enough that it engages the next gear.

If you try to shift at low revs using the clutch, you'll find that often it doesn't want to go into the next gear until you start releasing the clutch. What's happening is that the end of those pins is pushing against the steel parts of the next gear that are in between the slots enough to get the gears going the same speed (but it's not properly in gear because the pin is sitting against the steel between the slots, not in the slots themselves). When you start releasing the clutch, that forces the gears to try to turn at different speeds, and then the pin drops into the next slot. This is also why a dog-engagement gearbox is difficult to shift at a standstill.

You can get dog-engagement gearboxes for some cars. They're used in rally racing.
 

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Discussion Starter #6
So basically the gear your in is always spinning the next gear and when you shift its already rpm matched?
Anyone have some better diagrams of this?
Sorry to keep asking questions about this but its really got me.
 

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Why is it that if you shift to neutral at say 50 mph and try to get it back into 2nd, that it grinds and you fight it to go into gear? I know its not good for the shift forks when this is done. I'm just curious because I have done it a couple of times.
 

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1killer1liter said:
Why is it that if you shift to neutral at say 50 mph and try to get it back into 2nd, that it grinds and you fight it to go into gear? I know its not good for the shift forks when this is done. I'm just curious because I have done it a couple of times.

Just rev it up and it goes in smooth as butter
 

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killer, the reason it grinds if you try to do what you described is that there is too much RPM difference between the two gears trying to engage. Instead of the pins popping into the slots, they skip over the slots, and the result is grinding. Try not to do this too much ... you'll wear out the ends of the dogs and edges of the slots, and then it will *always* grind.

kawiman88, your description is correct for a synchromesh but not for a dog-engagement box. With dog-engagement it will only reliably engage the next gear if the gear with the pins is NOT rotating the exact same speed as the adjacent gear with the slots. 'Course, if there is too much RPM difference, you get the situation that killer describes.

Whatever you do, don't do what I did to my FZR400 race bike gearbox a couple of years ago. I missed the 3-to-4 shift at 14,000 rpm on the back straightaway at Shannonville. The engine bounced off the rev limiter and then the transmission decided to engage 4th. Instead of engaging the gear, it sheared all three engagement pins clean off the gear. Bye-bye, 4th gear. Bottom end overhaul to fix that ...
 

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Thanks gofaster.
 
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